Date Published: June 23, 2014
Publisher: Public Library of Science
Author(s): Jillian L. Sanford, Sharon A. Barski, Christina M. Seen, Joseph C. Dickens, Vonnie D. C. Shields, Frederic Marion-Poll.
The interactions between insect repellents and the olfactory system have been widely studied, however relatively little is known about the effects of repellents on the gustatory system of insects. In this study, we show that the gustatory receptor neuron (GRN) located in the medial styloconic sensilla on the maxillary palps of gypsy moth larvae, and known to be sensitive to feeding deterrents, also responds to the insect repellents DEET, IR3535, and picaridin. These repellents did not elicit responses in the lateral styloconic sensilla. Moreover, behavioral studies demonstrated that each repellent deterred feeding. This is the first study to show perception of insect repellents by the gustatory system of a lepidopteran larva and suggests that detection of a range of bitter or aversive compounds may be a broadly conserved feature among insects.
The effects of insect repellents have been shown to elicit aversive behavior in numerous insect species through the olfactory system , . Three chemicals, in particular, DEET, IR35535, and picaridin, have been shown to be effective insect repellents , , , . A previous study performed on A. aegypti demonstrated these three insect repellents elicited responses from a gustatory receptor neuron (GRN) housed within labellar sensilla located at the tip of the proboscis . In this study, we were interested in determining if the same repellents elicited responses from a deterrent-sensitive GRN in a lepidopteran larva. N, N-diethyl-3-m-toluamide (DEET) is a popular insect repellent that is used in commercial bug sprays, including “Repel” and “Off” , . DEET is of particular importance because it is capable of repelling numerous insect vectors of harmful diseases. Over the last twenty years, great strides have been made to elucidate the mechanisms of action of insect repellents , . While it is known that olfaction plays a major role in mediating the behavioral effects of DEET and other insect repellents, recent research on two selected adult dipteran species clearly demonstrated that repellents may also act through the gustatory system , . These authors demonstrated that two adult dipteran species, the vinegar fly Drosophila melanogaster and the yellow-fever mosquito Aedes aegypti have a gustatory receptor neuron (GRN) housed within the labellar sensilla sensitive to DEET (D. melanogaster) and two other insect repellents, IR3535 and picaridin (A. aegypti). Lee et al.  also showed that DEET deterred feeding in D. melanogaster. Additionally, IR3535 and picaridin have been shown to elicit aversive feeding behavior in other mosquito species, such as the black salt marsh mosquitoes, Ochlerotatus taeniorhynchus. Another study demonstrated behavioral responses of larvae of the malaria vector mosquito Anopheles gambiae to DEET . Nothing is known, however, about the detection of insect repellents by the gustatory sensilla in larvae of other species, such as lepidopteran caterpillars. The gypsy moth larva, Lymantria dispar, is a species of horticultural importance as it is notorious for defoliating numerous plant species, particularly forest, fruit, shade and ornamental trees , .
Although the interactions between insect repellents and the olfactory system have been widely studied (see reviews in insect olfaction) , , relatively little is known about their interactions with the gustatory system. While recent research has demonstrated a direct link between insect repellents and the adult gustatory system , , nothing was known about how these repellents interact with the larval taste system. Here we demonstrate that insect repellents are detected by a specific GRN in the larva of the L. dispar and the presence of the repellents deters feeding.